The characteristic curve of a real differential amplifier used at the input stage of an oscilloscope conventionally is biased corresponding to the zero point. The signal at the output port of the amplifier differs from zero, if the two input ports of the amplifier are connected to ground potential. The characteristic of the real amplifier in this case corresponds to an ideal amplifier which is supplied by an offset voltage at its input. Unbalances between the transistors at the input stage of the real differential amplifier cause such an offset voltage.
For compensating such an internal offset voltage an external offset voltage with an identical absolute value and an opposite sign in comparison to the internal offset voltage can be supplied to an input port, preferably to the inverting input port, of the real differential amplifier.
Additionally, a DC signal component in the measurement signal supplied to the non-inverting input of the differential amplifier in the oscilloscope can be shifted in the display by such an external offset voltage supplied to the inverting input port.
EP 1 688 748 A1 shows a conventional circuit for supplying an external offset voltage to a differential amplifier compensating both a DC signal component in the measurement signal and an internal offset voltage.
FIG. 1 shows the main features of this state of the art. The measurement signal Vin is supplied via a frequency dependent voltage divider comprising a parallel circuit of a resistor R1 and a capacitor C1 in series to a parallel circuit of a resistor R2 and a capacitor C2 to a non-inverting input of and differential amplifier 1. The inverting input of the differential amplifier 1 is supplied by an offset voltage Voffset which is generated at a first terminal 2 of an adjustable offset voltage generator 3. The second terminal 4 of the adjustable offset voltage generator 3 is connected to ground. The dynamic voltage range of the signal Vout at the output of the amplifier 1 is determined by the upper supply voltage Vcc provided at the first terminal 5 of the upper supply voltage source 6 and the lower supply voltage Vee provided at the first terminal 7 of the lower supply voltage source 8. The second terminal 9 and 10 of the upper supply voltage source resp. lower supply voltage source 6 resp. 8 are each connected to ground potential.
The signal Vout at the output of the amplifier 1 corresponds to the difference between the measurement signal Vin amplified by the amplification factor α of the frequency dependent voltage divider and the offset voltage Voffset according to equation (1).Vout=α·Vin−Voffset  (1)
The circuit in FIG. 1 contains following disadvantages:
The measurement signal Vin and the offset voltage Voffset are limited by the common-mode input signal range of the differential amplifier 1. The common-mode input signal range of the differential amplifier is limited by the amplification factor of the differential amplifier and the difference between the upper supply voltage Vcc and the lower supply voltage Vee. Additionally, the non-linear characteristic of the differential amplifier 1 reduces the common-mode input signal range. Consequently, the limited common-mode input signal range reduces the maximum possible difference between the measurement signal Vin and the offset voltage Voffset. Thus, arbitrarily high values for the measurement signal Vin and for the offset voltage Voffset cannot be realized.
The offset voltage Voffset changes the common-mode input voltage of the differential amplifier 1 and thus shifts the measurement signal Vin amplified by the differential amplifier in the dynamic voltage range at the output of the differential amplifier which is determined between the upper supply voltage Vcc and the lower supply voltage Vee. Consequently, the DC operation point of the differential amplifier is changed in direction to the non-linear characteristic of the differential amplifier resulting in a worse linear behavior. Correspondingly, classification numbers characterizing the non-linear behavior of the differential amplifier such as THD (i.e. total harmonic distortion), IP2 (i.e. intercept point second order), IP3 (i.e. intercept point third order) and so on increase.
The amplifier used for DC compensation and for internal offset voltage compensation has to be a differential amplifier. The behavior of a differential amplifier, for example the power consumption and the dynamic voltage range, is comparatively worse as a single port amplifier. For some applications a differential amplifier is thus disadvantageous.
Currents caused directly or indirectly by the external offset voltage generator can run back via signal paths in the amplifier, the connecting lines, the frequency dependent voltage divider and the DUT to the external offset voltage generator. This results in unwanted voltages across the connecting lines, the frequency dependent voltage divider and the DUT shifting the DC operation point of the differential amplifier 1 and thus producing additional noise in the output signal of the differential amplifier.
Therefore, one object of the invention is to develop a circuit for compensating both an offset voltage in an amplifier or a DC component in a signal supplied to the amplifier which overcomes the above mentioned disadvantages.